The fabrication of multifocal ophthalmic lenses by fusing one or more segments or buttons of a glass with a high refractive index into a cavity provided in a blank of pressed crown glass is well known. In common current practice, the blank is formed of a crown glass with a refractive index of 1.523 and the segment or button is constituted of a glass with a higher index in the range of 1.58-1.71, depending upon the degree of visual correction required for the finished multifocal lens. Glasses designed to comprise the segment or button of such multifocal lenses have been described, for example, in U.S. Pat. Nos. 3,902,910 and 4,211,569. All of the glasses disclosed in those two patents have refractive indices lower than 1.71.
It would be advantageous to utilize segment glasses with higher refractive indices when a very high degree of visual correction is required. In effect, this would permit reducing the depth of the cavity which receives the segment and, consequently, reducing the total thickness of the multifocal lens and, therefore, its weight.
The object of the invention is to provide glasses designed for the formation of segments for multifocal ophthalmic lenses with a refractive index n.sub.D ranging from 1.745-1.771, while having an Abbe number .nu..sub.D which is sufficiently high that undesirable iridescence at the edges of the lenses is precluded.
In addition, since the segment fusing operation necessarily implies the fusing of the latter to the major lens, the segment glass must have physical properties which are similar to those of the major portion, viz., the crown glass. In particular, the coefficients of thermal expansion of the segment and major portion must be closely approximating such that the seal between the two glasses resulting from the fusing step demonstrates a very low level of stress. The non-photochromic crown glass being considered here, called .-+.Blanc de Lunetterie" and manufactured by Corning Glass Works, Corning, New York, exhibits the following properties: a softening point of 707.degree. C., a strain point of 505.degree. C., an annealing point of 538.degree. C., and a coefficient of thermal expansion over the range of 25.degree.-300.degree. C. of 94.times.10.sup.-7 /.degree.C. Consequently, the segment glass must have a softening point of approximately 640.degree.-700.degree. C. and a coefficient of thermal expansion over the range of 25.degree.-300.degree. C. of approximately 90-98.times.10.sup.-7 /.degree.C. The stress between the crown glass and the segment glass, as evidenced by birefringence, is evaluated, for example, with a Babinet compensator. It is generally considered satisfactory when a seal with a crown glass demonstrates a birefringence measurement between -100 and +100 nm/cm. Preferably, this birefringence will lie between -50 and +50 nm/cm.
Besides the necessary optical properties and the proper viscosity and thermal expansion characteristics, the segment glass must display the chemical durability required for an ophthalmic glass. An evaluation of chemical durability is defined in terms of weight loss and visual appearance after attack. The test currently utilized for ophthalmic applications, viz., the A.O. test, consists of immersing a glass sample, the surfaces of which have been polished, for 10 minutes into a 10% by weight aqueous solution of hydrochloric acid (HCl) at 25.degree. C. and then measuring the weight loss (in mg). A weight loss per unit surface area is thereafter calculated, generally in terms of mg/cm.sup.2. The glass surface is also examined visually to detect the presence of defects (iridescence, flaws, dulling, etc.). For the glasses of the present invention, the limit which is not to be exceeded is exemplified by the chemical durability of a segment glass manufactured by Corning Glass Works, Corning, New York, which has been acknowledged to be satisfactory for ophthalmic applications. That glass exhibits a refractive index of 1.681 and is marketed under the code "Ba 683". That glass demonstrates a weight loss of 0.68 mg/cm.sup.2 in the A.O. test accompanied with marked surface dulling.